Environmental Engineering Reference
In-Depth Information
or
σ
v
γ
b
z
(3.20)
where
γ
b
γ
t
γ
w
is the effective or submerged soil weight.
In calculations, therefore, above the groundwater level, the effective soil weight is the
total weight
γ
t
, and below the groundwater level (or any other water surface), the effective
soil weight is the submerged soil weight
γ
b
.
Prestress in Soil Formations
General:
Soils compress naturally under the weight of overlying materials or some other
applied load, resulting in strength increase over values inherent as deposited, or shortly
thereafter. Three categories of prestress are defined according to the degree of compression
(termed consolidation) that has occurred.
Normally consolidated (NC):
The soil element has never been subjected to pressures
greater than the existing overburden pressures.
Overconsolidated (OC):
The soil element has at some time in its history been subjected to
pressures in excess of existing overburden, such as resulting from glacial ice loads,
removal of material by erosion, desiccation, or lowering of the groundwater level.
Underconsolidated (UC):
The soil element exists at a degree of pressure less than existing
overburden pressures. This case can result from hydrostatic pressures reducing overbur-
den load as illustrated in Figure 3.20. Such soils are normally relatively weak. Weakening
of strata can also occur due to removal of a cementing agent or other mineral constituents
by solution.
Principal Stresses and the Mohr Diagram
Importance
Fundamental to the strength aspects of geologic materials are the concepts of principal
stresses and the Mohr diagram on which their relationships may be illustrated.
Rainfall
Solution of
cementing agent
Hydrostatic uplift
Cementing agent in
granular soils
Clay
Cementing agent dissolved
leaving loose structure
Sand
Hydrostatic head removes some effective overburden
pressure; load on soil can be less than existing
overburden load (under consolidated)
FIGURE 3.20
Soil profile weakening processes.
